The invention is in the field of electronic reproduction technology and is directed to an engraving element for engraving printing forms for rotogravure as well as to a damping mechanism for an engraving element.
In an electronic engraving machine, an engraving element with an engraving stylus as a cutting tool moves along a rotating printing cylinder in the axial direction. The engraving stylus controlled by an engraving control signal cuts a sequence of cups arranged in a rotogravure raster into the generated surface of the printing cylinder. The engraving control signal is formed by superimposition of a periodic raster signal with image signal values that represent the hues to be reproduced between “black” and “white”. Whereas the raster signal effects a vibrating lifting motion of the engraving stylus for generating the rotogravure raster, the image signal values control the cutting depths of the engraved cups in conformity with the hues to be reproduced.
DE-A-23 36 089 discloses an engraving element with an electromagnetic drive element for the engraving stylus. The electromagnetic drive element is composed of a stationary electromagnet charged with the engraving control signal in whose air gap the armature of a rotatory system moves. The rotatory system is composed of a shaft, the armature, a bearing for the shaft and of a damping mechanism. One shaft end merges into a stationarily clamped resilient torsion rod, whereas the other shaft end carries a lever to which the engraving stylus is attached. An electrical torque is exerted on the armature of the shaft by the magnetic field generated in the electromagnet, this electrical torque being opposed by the mechanical torque of the torsion rod. The electrical torque turns the shaft around its longitudinal axis by a rotational angle proportional to the respective image signal value, turning this from a quiescent position, and the torsion rod guides the shaft back into the quiescent position.
Due to the rotational movement of the shaft around the longitudinal axis, the engraving stylus executes a lifting motion directed in the direction onto the generated surface of the printing cylinder, this respectively defining the penetration depth of the engraving stylus into the printing cylinder.
The damping mechanism serves the purpose of defined damping of rotational oscillations and transverse oscillations of the rotatory system and, thus, for damping the movement of the engraving stylus.
Given, in particular, sudden changes in the image signal values at steep density transitions (contours), the engraving stylus can exhibit a faulty activation and deactivation behavior that is essentially dependent on the degree of damping achieved in the damping mechanism. The consequence of a faulty activation behavior of the engraving stylus is engraving errors on the printing cylinder or disturbing changes in hue in the print.
Given inadequate damping of the rotatory system, disturbing multiple contours arise at density discontinuities due to over-shooting of the engraving stylus. Given too great a damping of the rotatory system, the engraving stylus cannot follow fast enough at steep density transitions, and the rated engraving depth is only achieved or left at a distance following the density discontinuity, as a result whereof steep density discontinuities are reproduced in unsharp fashion.
Moreover, a high temperature and long-term stability of the degree of attenuation are required.
The quality in the engraving of printing forms is thus influenced substantially by the degree of damping of the engraving element.
In a first exemplary embodiment, the damping mechanism disclosed by DE-A-23 36 089 is composed of a damping element connected to the shaft of the engraving element that immerses into a stationary damping chamber filled with a damping grease as damping agent. The damping element is designed as a circular damping disk or has at least one damping wing. A damping grease loses its damping properties over time due to the mechanical stressing and thus does not exhibit the required long-term stability.
In a second exemplary embodiment, the damping mechanism disclosed by DE-A-23 36 089 comprises two or more identical damping elements axially symmetrically at the circumference and stationarily connected at the outside to a seat, these damping elements residing under pre-stress in the radial direction. The damping elements are composed of an elastic-plastic synthetic, for example of a fluor-elastomer. The degree of attenuation that can be achieved at the moment with an elastic-plastic synthetic is dependent on the respectively preceding shaping. This “memory” effect disadvantageously leads to the fact that the engraving stylus achieves and in turn departs the rated engraving depth only with a disturbing delay.
In order to achieve a higher engraving speed, efforts have been undertaken to increase the engraving frequency, i.e. the frequency of the raster signal. A higher engraving frequency, however, leads to an increased production of heat in the engraving element. The employment of damping elements composed of an elastic-plastic synthetic has the further disadvantage that this does not eliminate the heat fast enough, this potentially leading to a modification for the degree of damping and, thus, to disturbing engraving errors.
U.S. Pat. No. 4,357,633 recites another electro-mechanical engraving element having a damping mechanism. The damping mechanism is composed of a circular damping disk connected to the shaft and of a stationary, annular bearing disk between which damping elements composed of an elastic, non-compressible material are arranged.
U.S. Pat. No. 4,123,675 discloses a damping mechanism for a stepping motor drive, whereby a magnetic disk with high inertia floats in a housing filled with a ferro-fluid. The housing is rigidly connected to the shaft of the stepping motor, i.e. it turns together with the stepping motor, and the friction between the inside housing wall and the inertial disk effects the damping.